An optical symbology is an information encoded symbol that is only readable by an optical sensing mechanism through a decoding process. For years, the bar code has been promoted as a machine readable symbology. A bar code is a one-dimensional symbol that contains a unique serial number encoded in black and white bars. As the demand for the information-based technologies grows, a number of two-dimensional symbologies, either in the form of a stacked bar code or a matrix, have been introduced to accommodate the need for storing more data information. This type of symbology is also referred to as a dense code. One of them, the MaxiCode symbology, is described in U.S. Pat. Nos. 4,874,936 and 4,896,029.
Generally there are two types of reading devices for symbologies, laser-based readers and devices based on Charge Coupled Device (CCD) cameras. The first type offers long range reading and does not -have focus problems while the second type offers cost-effective means and is more rigid in hostile environments.
Two-dimensional symbologies such as the MaxiCode cannot be acquired and decoded by the laser spot scanners typically used to scan single line bar codes. Attempts have been made to modify such devices to perform raster scanning operations on stacked bar codes and two-dimensional codes. However, this approach is too lengthy for practical high speed applications. A faster method is to acquire an image of the entire two-dimensional symbology using an electronic camera. CCD sensor arrays of the type used in video cameras are suitable. They consist of a matrix of "pixels" each of which stores a cumulative charge according to the amount of light which strikes the pixel. The pixel values are read out, converted and stored in a corresponding matrix in a digital memory for evaluation and processing.
One of the problems with a CCD camera-based symbology reader is its limited depth of field. To ensure the focus of a symbol to be decoded, a shroud for providing a fixed focal length is often used. There are many contact readers in the art which use a physical hood or a shroud to define a fixed object distance equal the focal length of the CCD camera. However, the contact of the shroud with the symbol results in much inconvenience in practice, especially in handheld applications. Furthermore, contact with the object bearing the symbol is simply not possible in over-the-belt applications.
To solve the focusing problem, attempts have been made to provide an automatic focus adjustment. For example, U.S. Pat. No. 5,308,966 to Danielson and et al. discloses a handheld CCD camera-based bar code reader employing a distance measuring system that uses an ultrasonic signal source to adjust the focusing mechanism. The distance measuring system detects the distance between the camera and the target to be imaged and then sends a signal to the camera mechanism to reposition some mirrors to alter the optical path so as to change the focal length to match the object distance.
Distance measurement based on the alterations in the received signal reflected from a target has been much explored in photographic camera technology. For example, an auto focus circuit may detect high frequency elements from the video signal, and focus to the subject with much brightness and contrast in the measuring area of the center screen. However, when such systems are provided in a camera that also has a zoom lens, the auto focus stops working properly inside a minimum distance, which increases as the zoom lens is moved toward the telephoto end of its range of movement. Also, if the object distance is beyond a certain range, measurement errors occur due to the weak reflected signals. Moreover, the reflected signal is subject to degradation caused by the surface characteristics of the target.
Generally, the image resolution varies with the object distance. The further the object is from the lens, the lower the resolution. In many situations, when an object is far away from a camera, the resolution may be low to the point of just blurring signals. On the other hand, the nearer the object is to the lens, the larger the number of dots per inch (dpi) on the object the detector can capture. Eventually, the resolution may become high to the point of being useless. Furthermore, software to decode dense codes must be more complex and will operate more slowly if the resolution of the image varies from object to object. Therefore, there has been a need in the art for systems that capture images of objects in the same resolution independent of the distance of the objects from the camera.
The concept of auto zoom has been developed for video cameras which have auto focus and zoom lenses. Various auto zoom methods have been devised to keep the image size constant while the object distance varies. U.S. Pat. No. 5,113,214 describes an optical zoom lens system which provides auto focus based upon detecting characteristics of the signal derived from a CCD. Auto zoom is achieved by first moving the auto focus lens to a predetermined position at which it remains fixed. Then the auto focus circuit sends a signal to adjust the zoom lens in a manner which both adjusts the focus and varies the size of a subject so that size of the subject is always constant regardless of changes in the object distance. This system has the disadvantage of requiring extra controls for both the auto focus lens and the zoom lens, and the auto focus lens is not allowed to work normally in the auto zoom mode. The system of this prior patent perhaps disables the normal auto focus function to avoid the auto focus minimum distance problem described above which occurs when the zoom moves too far toward the telephoto.
U.S. Pat. No. 5,173,807 also describes an auto focus and auto zoom system. An infra-red range finder provides the object distance to an auto focus lens controller. The zoom lens control monitors the auto focus lens position and follows the detected movement of the auto focus lens to make the size of an object in the picture plane substantially constant. This system has the disadvantage of requiring a separate range finder and a special set of pulse generators for knowing the amount of rotation of the auto focus and zoom motors.
There remains a need in the art for an auto zoom system that does not require a separate range finder, that allows an auto focus system to function normally, and that is not subject to problems resulting from the limitations of auto focus systems which operate based on the alterations in the received signal.